It has been proposed to utilize metals for the construction of artificial prostheses and orthopedic and dental devices. The biological inactivity of metal surfaces, however, renders it impossible to achieve cement-free implanation of metal prostheses since bone tissue will not bond or grow thereon.
Various biologically active glasses have recently been introduced for the preparation of artificial prostheses. It is known that bone and other biological tissue will bond to or grow on these biologically active glasses. However, the strength characteristics of the glasses are such that it is impossible to construct sufficiently strong orthopedic or dental devices therefrom.
It has been suggested to overcoat metal substrates with biologically active glasses to provide sufficiently strong orthopedic or dental devices capable of bonding to bone tissue. However, there are numerous difficulties associated with bonding such glasses to metal surfaces. For example, the thermal coefficients of expansion of the metal and glasses are so dissimilar at both the melting and softening points of the glasses that cooling the coated metal substrate results in extreme thermo-mechanical stresses in the glass and metal layers which, when relieved, cause cracks, etc., in the glass coating.
Prior art methods of coating metals with glasses necessarily require the utilization of glasses and metals which have substantially identical thermal coefficients of expansion and which can withstand those temperatures at which the glass flows. The prior art methods are particularly disadvantageous where it is desired to coat a particular metal with a particular high melting glass in order to obtain a product with specific properties. The problems which normally arise in this connection are:
(1) Excessive scaling of the metal substrate at the elevated temperatures:
(2) Loss of compositional control of the glass through long firing times;
(3) Excessively high diffusion of metal ions into the glass bulk;
(4) The virtual impossibility of matching the thermal coefficients of expansion due to the fact that the choice of glass and metal substrate are fixed because of the desired application of the product.
It is possible to coat metal substrates with mismatched (thermal coefficients of expansion) glass by flame spraying; however, this method gives rise to other disadvantages, namely, high volatilization and loss of glass components, limited choice of glass compositions, extremely high working temperatures and adverse surface reactions on the metal substrate. It has been proposed to utilize biologically active glasses and metals having similar thermal expansion coefficients; however, this approach drastically limits the number and variety of permissible combinations.
It has also been proposed to coat metal surfaces by applying powder mixtures of the glass thereto and utilizing long firing times and multiple coatings to produce transition layers between the metal and glass having gradient coefficients of expansion to thereby relieve the thermo-mechanical stresses. Obviously, however, this is an expensive and time consuming procedure which, by its very practicality, severely limits the nature of the ultimate product. German Patent DT 2326100 B2 describes a glass coated material useful as a prosthetic device. However, the German patent requires an intermediate layer of low reactivity glass between the metal substrate and the biologically active glass.
It is an object of the invention to provide an inexpensive method for bonding biologically active glass to metal for the formation of artificial prostheses and orthopedic and dental devices.